This invention relates to a system for automatically controlling the shifting of a multi-ratio power transmission, such as may be used in vehicles to couple the engine to the wheels or other driving members of the vehicle.
For optimum vehicle performance, the engine should be operated to deliver maximum horsepower under all operating conditions. Particularly with Diesel engines, this requires that the engine be operated within a relatively narrow speed range, since the efficiency of the engine will drop off rapidly when operated outside of such range.
The typical transmission is designed to enable a broad range of vehicle ground speeds to be realized with the engine operating with its narrow speed range wherein the delivered horsepower is at or near maximum.
A common form of transmission, particularly for heavy-duty vehicles such as earthmoving equipment, utilizes a combination of mechanical and torque converter drive. At very low vehicle speeds, such as when the vehicle is operating in reverse or first forward gear, a torque converter is connected into the power path since it has a relatively broad torque range capability. With the torque converter in the power path, the engine can operate within a narrow speed range, and the power delivered through the torque converter will remain reasonably high for a relatively broad range of vehicle speeds. The fluid losses through a torque converter are substantial, but such losses are normally acceptable in these low ranges of vehicle speeds.
At high vehicle speeds the transmission is operated in direct drive to increase the efficiency of power throughput. For direct-drive operation, a large number of drive ratios must be provided, with the step ratios between successive speed ratios being small and preferably uniform.
Any direct-drive vehicle transmission represents a compromise. If too few drive ratios are provided, the range of engine speed for given drive ratio is undesirably broad. If too many drive ratios are provided, the bulk, complexity and cost of the transmission increases, as well as the inertia and friction losses. In addition, the control is complicated in that much more shifting is required as the vehicle speed changes. In general, as many drive ratios are provided as is practicable.
The design of a transmission should preferably be such that the range of engine speed is approximately the same for each of the drive ratios in which the vehicle is operating, both to increase the efficiency of the system and also to make the shift points more uniform. Practical considerations, however, often dictate a certain degree of non-uniformity of drive ratios.
Regardless of how well designed a transmission may be, its design will only determine the maximum efficiency that can be obtained. To achieve the maximum efficiency, up or down shifting from a particular gear, or drive ratio, must be done at definite optimum engine speeds for that particular gear.
Observations in the field show that only highly skilled operators shift gears at the proper engine speeds. Less skilled operators (and distracted skilled operators) shift gears either too early or too late, so that the engine is forced into an inefficient speed. In addition to increasing the operating costs, inefficient operation can cause undue deterioration of the equipment. Also, when auxiliary equipment, such as used on earthmoving equipment, is powered by the engine, operation of the engine at an inefficient speed can adversely affect the functioning of the auxiliary equipment either by increasing the time to complete an operational cycle or by reducing the power to such equipment to a point wherein it will not function satisfactorily.